Drug Sniffing Dogs and Quantifying Probable Cause, Redux

My first post on the reliability of drug detection dogs was linked to by EvidenceProf Blog, where Colin Miller makes an excellent point about why quantifying probable cause may be more feasible than quantifying proof beyond a reasonable doubt. Although an artificial imposition of a numeric value on the proof beyond a reasonable doubt standard will likely confuse jurors, writes Miller, we believe in many contexts that judges are better equipped to navigate complex evidentiary matters.

Since judges- not jurors- decide whether probable cause justifies a particular search, quantifying probable cause might not unearth such a parade of horribles, the fear of which has imprisoned the probable cause standard in fuzziness for so long. We may need to quantify probable cause to decide if a positive alert by a drug sniffing dog gives the police enough suspicion to conduct a full search of a person’s car, or suitcase, or of the person’s person.

Let me be clear. I am not arguing (at least for now) that every probable cause determination should be calculated according to a mathematical algorithm where the evidence has to yield a certain likelihood of criminal behavior (40%?, 50%?, 51%?) before probable cause is satisfied. However, if the Supreme Court affirms the Florida Supreme Court’s holding that evidence of a dog’s false positive rate must be admitted as part of the probable cause inquiry, courts will have to grapple with some quantification of probable cause. Unless the Supreme Court decides that a positive alert by a drug detection dog is never, on its own, sufficient to establish probable cause, courts will confront situations where a dog’s positive alert is the sole reason to suspect someone of possessing drugs. This could occur at an airport, or at a drunk driving checkpoint, or anywhere where drug dogs are permitted to routinely sniff individuals without suspicion. In that case, courts will have to decide if the dog sniff alone is sufficient to establish probable cause.

As a result, courts may have to determine, as a matter of law, whether a particular false positive rate is too high to satisfy probable cause. This presents several problems, mentioned in the comments to my previous post on drug sniffing dogs. A false positive rate measures the likelihood that a dog will alert to drugs, given that there are no drugs in the vicinity. (Probability of alert, given absence of drugs). To determine whether an alert is insufficient to satisfy probable cause, we’d have to know the likelihood that there are no drugs in the vicinity, given a positive alert. (Probability of absence of drugs, given alert). These two probabilities are not the same, and the former can be converted into the latter if the police have information on the base rate of drugs on individuals in a particular area.

This does pose a bit of a problem, but does not mean that we should simply presume that because a drug detection dog has received training, its positive alert automatically establishes probable cause. (I fear the Supreme Court may decide something to this effect in Florida v. Harris.) Nor do I think we should resign ourselves to the fact that a positive alert can never satisfy probable cause. Instead, perhaps we can standardize dog training programs and create simulated situations where we know the base rate of drugs (because the police have created the simulation) and can convert the false positive rate into a probability that a given individual will not possess drugs, given a positive alert. At that point, this probability can be converted directly into probable cause, and courts will need to know what likelihood of criminality meets the probable cause standard.

Quantifying probable cause in this way does not mean an abandonment of the totality of the circumstances test. Police officers can still rely on their instincts when there are various indicators, in addition to a dog’s positive alert, that a suspect is engaging in criminal behavior. Courts can still weigh all of these indicators in conjunction with a positive alert, and, in that case, we might not need to quantify probable cause or decide if a particular dog is too unreliable. But, when the only indication of probable cause is an alert by a dug detection dog, courts need to decide whether probable cause means that an officer must be 40%, 50%, or 51% sure that a particular vehicle contains drugs.